Abstract

The N–H…π hydrogen-bonded (H-bonded) structures of pyrrole (Py) and N-methylpyrrole (NMPy) binary clusters have been studied by IR cavity ringdown spectroscopy and density functional theory calculations. The Py1-NMPy1 cluster has an “L-shape” structure, which is formed by an ordinary H-bond between a N–H donor of Py and a π-electron cloud acceptor of NMPy. The Py2-NMPy1 cluster has a “Cyclic” structure, which is also formed by ordinary N–H…π H-bonds as well as the weak C–H…π H-bond between the methyl CH group and the π cloud acceptor of Py. On the other hand, the Py1-NMPy2 cluster shows an extraordinary structure, in which the single donor NH group is surrounded by a three-dimensional H-bond acceptor formed by two aromatic π electron clouds. We call the Py1-NMPy2 cluster as the “Fish-Bite” structure. The Py1-NMPy2 cluster exhibits a redshifted NH stretch by 157 cm−1 from the Py monomer, which is larger than 94 cm−1 of the Py1-NMPy1 cluster. However, both Py1-NMPy1 and Py1-NMPy2 clusters have calculated IR intensities of 169 and 163 km/mol, respectively. This result indicates that not only the N–H…π H-bonds but also the dipole-dipole interaction between Py and NMPy contributes to the Fish-Bite Py1-NMPy2 cluster formation.

Received 16 May 2012Accepted 01 August 2012Published online 20 August 2012

Acknowledgments:

Y.M. thanks Professor Martin A. Suhm at Universität Göttingen for fruitful discussion with his unpublished results. This work is supported by a Grant-in-Aid for Scientific Research on Priority Areas “Molecular Science for Supra Functional Systems” [477] (Grant No. 22018025) from MEXT, Japan.